1. Field of Invention
This invention relates generally to the field of integrated circuit manufacturing and in particular to a method for focusing the projection step and repeat photolithography printer employed in a photoresist masking step of an integrated circuit manufacturing process.
2. Description of the Prior Art
A manufacturable, high resolution photoresist process is essential for the development of VLSI circuits. Single layer photoresist processes known in the art are capable of fine resolutions when used with smooth, low reflectivity substrates. Unfortunately, the process of manufacturing integrated circuits creates a complex multi-layered surface of widely varying reflectivities. In response to these problems, several complex photoresist processes have been developed.
One such process is the portable conformal mask process. This process employs a two layer photoresist. The first step of the process is to apply the photoresist layers. The first photoresist layer is typically a polymethyl methacrylate (PMMA) photoresist and is applied in a manner which planarizes the substrate. The second photoresist layer is a standard photoresist layer and is applied in a manner which forms a layer having uniform thickness. Then, the second photoresist layer is exposed using a projection step and repeat photolithography printer. This printer, also referred to as a mask aligner or wafer stepper, exposes the second photoresist layer with monochromatic or quasi-monochromatic light through a master reticle containing the pattern to be constructed. The second photoresist layer is then developed. The developed second photoresist acts as the mask for the first photoresist layer as it is exposed. The first photoresist layer is then developed to form a photoresist mask and the second photoresist layer is usually removed. With the photoresist mask in place, the chemical processing begins and the necessary circuit formations are constructed.
A critical step in the above process is the exposure of the second photoresist layer by the projection printer. The master reticle used by the projection printer to expose the second photoresist layer is typically many times the size of the image to be formed on the photoresist, consequently, a reduction lens is employed to reduce the size of the projected image. In order for the fine lines, typically one to two microns in width, to be formed, the projected image must be precisely focused on the top of the second resist layer. The top of the second layer is referred to as the image plane. Since the images involved are so small, optimum focus is maintained during the exposure of the photoresist layer through the use of an automatic focusing system which compensates for variations in the thickness of the photoresist layers and a standard wafer.
Typical automatic focusing techniques employ an opto-electrical system which reflects an optical signal off the image plane to determine focus and adjusts the distance between the reduction lens and the image plane accordingly thereby focusing the image. A typical automatic focusing system comprises an optical transmitter, an optical receiver and control circuitry. The optical transmitter comprises one or more infra-red light emitting diodes which generate an optical signal with a wavelength of 800 nm to 900 nm. The optical signal is typically modulated or the multiple diodes are sequenced to generate a phase shifted signal. This optical signal reflects off the surface of the wafer into the optical receiver where the optical signal is converted into an electronic signal. The electric signal is fed into the control circuit to generate an error signal which is used to focus the reduction lens.
Ideally, the optical signal entering the optical receiver is generated by reflecting only off the image plane. However, despite a shallow angle of incidence, a portion of the optical signal will enter the second photoresist layer to be reflected by the boundary between the first and second photoresist layers and by circuit formations on the surface of the wafer. These reflections ultimately reach the optical receiver where they are also converted to electronic signals and appear as noise on the main focusing electronic signal. This noise causes the focusing system to incorrectly focus the reduction lens which inturn causes the resist to be improperly exposed. Poor focus is a major source of defects in integrated circuit manufacturing process.
Prior art solutions have attempted to increase the signal to noise ratio of the optical signal by using more powerful light emitting diodes with a narrower emission spectrum, by reducing the diameter of the optical signal or by increasing the angle of incidence. Each of these attempted solutions has failed to resolve the problem of maintaining the very narrow line widths with the high degree of uniformity and accuracy demanded by VLSI integrated circuits.